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Polymers - ethene

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The double bond in an alkene (like ethene) can be broken open and joined to other molecules. A molecule of ethene can be joined to another molecule of ethene. In the process, the second ethene molecule has its double bond broken, and this may be added to a third molecule of ethene, and so on. Many ethene molecules can be joined together to form a polymer. The polymer is called poly(ethene) because it is made from ethene. Poly(ethene) is commonly called polythene. Polythene is an ICI trademark for poly(ethene). Ethene put under pressure and heated with a catalyst will polymerise to form poly(ethene). Note that there are no double bonds in the polymer. Poly(ethene) is an alkane. It is a saturated hydrocarbon. A polymer is often written in the form A polymer which is formed from monomers added together where no substance (other than the polymer) ...read more.


When the polymer is heated the crystals will melt, the material will become very soft and can flow slowly like a thick liquid. In this state the polymer can fill a mould and be cast into a shape. When the polymer cools down, new crystals can form between the chains and the new shape is fixed. The same polymer can be reheated and remoulded. Such polymers are called thermosoftening (meaning that they go soft when you heat them). The picture below shows the tangled polymer chains which have lined up and crystallized in the pink region. Some polymers do not form crystals. They soften when heated and harden when cooled down again. When cold, they are not crystalline but glassy. These polymers are also called thermosoftening. ...read more.


Products from plant material (wood, paper, cotton etc.) are biodegradable. When buried, bacteria and fungi break them down into useful nutrients for further plant growth. Nature recycles its own products! Polymers produce toxic materials (poisons) when burnt, in addition to the expected products from combustion of a hydrocarbon which are carbon dioxide, water, carbon monoxide and carbon (soot). Those which contain chlorine (PVC for example) also produce hydrogen chloride on burning. Those which contain nitrogen (nylon for example) produce hydrogen cyanide. Polymers are a fire hazard. Many people die from the smoke of burning polymers in house fires, long before the fire reaches them. Burning polymers is not a good way of disposing of them. Solutions. More and more polymers are being recycled. This is not as cost effective as recycling metals, but we don't want to live amongst piles of (unrotting) plastic. Research into biodegradable polymers will increasingly provide useful replacements for the main polymers of today. ...read more.

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